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What are the TCP or UDP ports used by Cisco Discovery Protocol (CDP)?

None of the above

161 and 162

1645 and 1646

20 and 21

Explanation

The Cisco Discovery Protocol (CDP) is a proprietary Data Link Layer protocol developed by Cisco Systems. It is used to share information about other directly connected Cisco equipment. Being a Data Link Layer protocol it does not use either TCP or UDP.
161 and 162 are used by SNMP.
1645 and 1646 were used for RADIUS.
20 and 21 are for FTP